Cardiovascular risk factors including blood pressure, lipoprotein concentrations, physical activity, and diet were assessed in 149 diabetic adolescents and 45 nondiabetic siblings. All diabetic subjects had had insulin-dependent diabetes mellitus (IDDM) for a minimum of 2 yr and were currently attending the Children9s Hospital of Pittsburgh Diabetes Clinic. For both boys and girls, cardiovascular risk profiles were mildly disturbed among diabetic subjects compared with nondiabetic siblings. These disturbances included higher systolic (P = 0.002) and diastolic (P = 0.024) blood pressures and higher HDL3 cholesterol concentrations. The diabetic girls showed higher total cholesterol concentrations during adolescence in contrast to the usual fall seen in nondiabetic adolescents (and evidenced in the siblings studied). In addition, the diabetic girls9 mean pulse rate was 12 bpm higher than that of the sibling girls, a finding not seen in the boys. Multiple linear regression analyses showed that neither glycemic control (worse in diabetic girls), diet, nor physical activity were important explanatory variables for any of the lipoprotein or blood pressure measures. These results suggest that the cardiovascular risk profile of diabetic girls may be relatively more disturbed than that of diabetic boys. This difference could not be explained by the slightly higher glycosylated hemoglobin levels in the girls. The loss of the sex differential in the risk for cardiovascular disease experienced by adults with IDDM may partly relate to these adolescent risk factor differences.